Sulphurous p-aramid yarn

ABSTRACT

A fiber of a para-aromatic polyamide polymer having a surprisingly higher modulus. This fiber has a quantity of sulphur bound to the aromatic nuclei of the polymer and wherein the quantity of nucleus-bound sulphur is 0.05-0.20% by weight. A process for the manufacture of para-aromatic polyamide fiber of increased modulus.

FIELD OF THE INVENTION

The invention pertains to a fibre of a para-aromatic polyamide polymer(p-aramid) comprising a quantity of sulphur bound to the aromatic nucleiof the polymer.

DESCRIPTION OF RELATED ART

Such fibres are known. For instance, EP 427 280 (Chiou) teaches apoly(paraphenylene terephthalamide) filament yarn having chemicallybound sulphur (sulphonic acid or sulphonate) groups. The yarn contains0.5 to 3.0% by weight of S thus bound. Also described is a process forthe manufacture of such a yarn. In this process the p-aramid polymer isnot spun directly from the conventional solution in concentratedsulphuric acid; rather, the substantially unsulphonated polymer is firstexposed to fuming sulphuric acid in a concentration of 100.5 to 102.5%for 1 to 3 hours at 70 to 80° C.

U.S. Pat. No. 4,075,269 and U.S. Pat. No. 4,162,346 (Jones) likewisedisclose sulphonating p-aramid in fuming sulphuric acid. In the processdescribed, poly(paraphenylene terephthalamide) containing 0.5 to 10% ofS in the form of sulphonic acid and/or sulphonate groups bound to thearomatic nuclei is spun into a yarn.

As background art is further mentioned EP 442 399 (Keil), whichdiscloses copolymers made from a mixture of monomers which comprises 7.5to 30 mole-% of an alkylsulphonyl substituted aryl monomer. Theexemplified sulphur content can be calculated to be about 1 wt. %.

Already in U.S. Pat. No. 3,767,756 (Blades) mention is made of the factthat fuming sulphuric acid can be used as solvent for spinningpoly(paraphenylene terephthalamide). However, a warning is given here tocarry out the treatment at a low temperature and in a short period oftime, to avoid a decrease of the inherent viscosity and the tensilestrength of the obtained fibres.

It is known that spinning p-aramid from concentrated sulphuric acid canproduce fibres with excellent mechanical properties. It is also knownfrom the aforementioned prior art that tensile strength in particularcan be improved by sulphonation. Further, it is known that a p-aramidfibre containing extra sulphur is better able to withstand a heattreatment. In this connection Chiou teaches good strength retentionafter heat ageing, while Jones teaches that a significant increase inthe tensile strength is found especially after a heat treatment.

In this familiar field of chemically (nucleus) bound sulphur-containingp-aramid fibres a novel, surprisingly improved fibre according to theinvention has now been found. The invention in this connection consistsof a p-aramid fibre of the aforementioned known type, with the quantityof nucleus-bound sulphur being 0.05 to 0.20% by weight, calculated ondry polymer. The fibre according to the present invention has anunexpectedly higher modulus. In addition, the fibre according to theinvention has a substantially higher tensile strength, even without aheat treatment.

Preferably, the para-aramid is made up wholly or for the most part ofpoly(para-phenylene terephthalamide), hereinafter referred to as PPTA,and the quantity of nucleus-bound sulphur is 0.09 to 0.20% by weight.Further preference is given to a sulphur percentage of 0.09 to 0.15% byweight.

The PPTA can be made in a known manner by reacting stoichiometricquantities of para-phenylene diamine (PPD) and terephthalic aciddichloride (TDC) in an appropriate solvent (notably CaCl₂ -containingN-methyl pyrrolidone). Suitable processes have been described in NL157327 and WO 95121883.

To make fibres having the desired properties, use is made of ananisotropic solution of the PPTA, which preferably has a relativeviscosity of at least 3.5, most preferably of more than 4.3. In thisprocess at least 14% by weight, preferably about 17.0-20.5% by weight,is dissolved in an appropriate solvent such as concentrated sulphuricacid. Such anisotropic solutions are fully known for p-aramid in generaland PPTA in particular. They can be prepared in a known manner, e.g.,with the aid of a freezing process as described in NL 7904495. Thespinning solution can be spun using a conventional dry jet-wet spinningprocess. Such processes are known from, int. al., U.S. Pat. No.3,767,756 and U.S. Pat. No. 4,320,081.

The aforementioned background literature, NL 157327, WO 95/21883, NL7904495, U.S. Pat. No. 3,767,756, and U.S. Pat. No. 4,320,081 is to beconsidered incorporated herein by reference.

To obtain the desired quantity of nucleus-bound sulphur (usually in theform of sulphone or sulphonate groups), the PPTA, or other p-aramid,after or instead of being incorporated into the usual concentratedsulphuric acid (generally at least 97%) is exposed for some time to verystrong sulphuric acid in a concentration of 99.8 to 100.2%. Sulphuricacid of the desired concentration can be obtained, e.g., by addingsulphuric acid having a concentration of 99.8% or higher to thesulphuric solution. For completeness' sake, it should be noted thatsulphuric acid having a concentration of more than 100% is known asoleum (fuming sulphuric acid, i.e., sulphuric acid containing free SO₃).Preferably, sulphuric acid in a concentration of at most 104.5% isemployed (20% oleum; although a higher percentage of oleum can be usedin theory, it will be more difficult then to keep the sulphur contentwithin the limits set by the invention). The treatment with strongsulphuric acid, which is preferably carried out at a temperature in therange of 70 to 90° C., generally lasts from 0.5 to 5 hours. In the caseof more than 20% oleum it may be necessary to opt for a shorterresidence time and/or a lower temperature. The skilled person will knowas a matter of routine how the oleum had best be added to the dissolvingand spinning process employed. When a freezing process is used to makethe spinning solution, as described in NL 7904495, the resulting sandyPPTA-solution is melted in an extruder prior to being spun. In thisprocess the PPTA is preferably sulphonated by injecting the oleum intothe extruder. According to the present invention, preferably an improvedversion of the freezing process is employed in which the PPTA mixed withconcentrated sulphuric acid in the freezing process is melted andfurther mixed in a kneader-mixer. In this process the polymer solutionpasses successively through at least a melting zone and a pressurebuild-up zone, and at least in the melting zone is kneaded as well asmixed. In such a process the addition of oleum preferably takes the formof injection into the melting zone. The improved process referred tohere is described in non-prepublished patent application NL 1000276,which should also be considered incorporated herein by reference.

It should be noted that in the prior art (Chiou, Jones) also there isquestion of treatment with sulphuric acid of a high concentration. Chiouteaches sulphonation of the polymer by dissolving it in 100.5 to 102.5%sulphuric acid and exposing it to this acid concentration for 1 to 3hours at a temperature of 70 to 80° C. In Jones there is question ofdissolving PPTA in concentrated (98 to 100%) sulphuric acid over aperiod of 14 to 50 hours at 75 to 95° C.

SUMMARY OF THE INVENTION

Unlike in this prior art, in the treatment according to the presentinvention, where the polymer is first dissolved in a conventional mannerin concentrated sulphuric acid followed by the addition of oleum, an0.05 to 0.20 quantity of nucleus-bound sulphur is obtained. In view ofthe surprising effect of this sulphur content on the modulus of the PPTAfibres, the invention also pertains to a process for the manufacture ofpara-aromatic polyamide fibres of increased modulus. The processcomprises spinning a sulphuric spinning solution of the para-aromaticpolyamide in a conventional manner by means of a dry jet-wet spinningprocess, and comprises at least one modulus-increasing treatment. Oneknown modulus-increasing treatment is to subject the fibre afterspinning to an additional hot drawing treatment. However, what isdesired is the possibility of increasing the modulus also without a heattreatment. In addition, it is desired to further increase the modulusobtainable by the heat treatment. The process according to the presentinvention solves this problem by means of a modulus-increasing treatmentconsisting of the above-indicated exposure of PPTA to fuming sulphuricacid prior to its being spun. When the fibre after being spun issubjected to the usual hot after-treatment, the modulus is increasedfurther. When a hot after-treatment is envisaged, the sulphur percentagepreferably is 0.09 to 0.20, more preferably 0.12 to 0.15. When no suchhot after-treatment is envisaged, the selected sulphur percentagepreferably is 0.09 to 0.15.

The fibres according to the present invention can then be made byspinning the above-described spinning solution treated with highlyconcentrated sulphuric acid in a conventional manner. Suitable spinningprocesses are known to the skilled person and require no furtherelucidation here. One pre-eminently suitable process is a dry jet-wetspinning process such as described in the aforementioned U.S. Pat. No.4,320,081.

In spinning processes such as indicated above usually endless filamentsare made which can be combined in a known manner into a filament yarn,or from which staple fibres or other types of short fibres can be made(e.g., by cutting up the filament yarn), which fibres can be spun in aknown manner to form a spun yarn. This is why within the framework ofthe present invention the term "fibre" stands for every conceivable typeof fibre irrespective of length, including staple fibres and endlessfilaments.

The fibres according to the invention can be employed in any knownmanner, but are preferably used in filament yarn.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further illustrated below with reference to thefollowing, unlimitative example and the accompanying graphs (FIG. 1 andFIG. 2).

FIG. 1 is a graph of the modulus versus the degree of sulphonation

FIG. 2 is a graph of the strength versus the degree of sulphonation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE

Use was made of a LIST DTB-60 kneader-mixer with four compartments and ashaft with mixing and kneading elements. In the fourth compartment thisshaft was equipped with a discharge screw element.

The wall of the second compartment (the melting zone) had a temperatureof 88° C., the wall of the third compartment (the degassing zone), aswell as that of the fourth compartment, had a temperature of 85° C. Theshaft with mixing and kneading elements was kept at a temperature of 90°C. and rotated at a speed of 40 revolutions per minute. Thekneader-mixer was kept under a vacuum of 30 mbar, and the degree offilling was set at 50%±5.

A solid polymer solution made up of 79.2% by weight of sulphuric acidand 20.8% by weight of poly(p-phenylene terephthalamide) was introducedinto the second compartment of the kneader-mixer via a vacuum dome, witha throughput of 40 kg/h. The residence time of the polymer solution wasabout 98 minutes±10. At the end of the second compartment, i.e., at theend of the melting zone, strong sulphuric acid was injected with the aidof a Lewa three-headed metering pump (supplied by: Geveke).

In a series of experiments in which the rate of injection was keptconstant, PPTA with different degrees of sulphonation was obtained byvarying the concentration of the strong sulphuric acid to be injectedfrom 99.8% to 104.5% (20% oleum).

Via a booster pump (Transmark™) and a transmission pump (Slack & Parr™)the molten spinning solution was passed to the spinning positions with athroughput of 40 kg/h and spun into filament yarns using a conventionaldry jet-wet spinning process at a rate of 400 m/min.

In FIG. 1 the connection is shown between the content of sulphurchemically bound to the aromatic nuclei of the poly(paraphenyleneterephthalamide) and the modulus, both for sulphonated fibres as such(referred to as "pre-yam") in the form of an endless filament yarncontaining 1000 filaments and for sulphonated yarn subjected to anadditional hot drawing treatment at a rate of 200 m/min, a temperatureof 400° C., a tension of 3000 cN (=18 cN/tex), and a residence time ofabout 6 seconds (referred to as "HM yarn").

In FIG. 2 the connection is shown between the content of sulphurchemically bound to the aromatic nuclei of the poly(paraphenyleneterephthalamide) and the modulus, again for both the pre-yarn and theHM-yarn as indicated above.

In FIGS. 1 and 2, respectively, the modulus and the tensile strength(both determined in accordance with ASTM D885) are shown plotted againstthe content of nucleus-bound sulphur. Said last content was measured bydissolving 100 m of yarn (1680 dtex), cut up into pieces, in 350 mlsulphuric acid of technical quality (95-97%) at a temperature of about80° C. As soon as the yarn was dissolved, the resulting solution wascoagulated in demineralised water, with filaments being formed. Thesefilaments were ground up in a blender into a comparatively fine, powderymaterial. This material was then boiled up for 15 minutes indemineralised water, filtered off, and transferred to a soxhlet set-up,where it was after-washed for 16 hours in demineralised water. Theobtained polymer was subsequently dried in a vacuum drying oven at atemperature of 80° C. Via XRF-S (also called XPS) the quantity of Spresent in this dry polymer as bound S was determined.

The surprising effect of the sulphur content according to the presentinvention is clearly shown in the figures.

What is claimed is:
 1. A fibre of a para-aromatic polyamide polymercomprising a quantity of sulphur bound to an aromatic nuclei of apolymer, wherein the quantity of nucleus-bound sulphur is 0.05 to 0.20%by weight, calculated on dry polymer.
 2. The fibre according to claim 1,wherein the para-aromatic polyamide is made up wholly ofpoly(para-phenylene terephthalamide).
 3. A fibre according to claim 2,wherein the quantity of nucleus-bound sulphur is 0.09 to 0.15% byweight.
 4. The fibre according to claim 1, wherein the para-aromaticpolyamide is an endless filament.
 5. A filament yarn comprising aplurality of fibres according to claim
 4. 6. A process for makingpara-aromatic polyamide fibres of increased modulus comprising spinninga sulphuric spinning solution of a para-aromatic polyamide by means ofan air-gap spinning process and at least one modulus-increasingtreatment, wherein the modulus-increasing treatment comprises contactingthe para-aromatic polyamide with sulphuric acid in a concentrations of99.8-100.7% for 0.5-5 hours at 70-90° C. prior to being spun to obtain apara-aromatic polyamide fiber wherein the quantity of nucleus-boundsulfur is 0.05 to 0.20% by weight calculated on dry polymer.
 7. Theprocess according to claim 6, wherein the modulus-increasing treatmentis carried out by adding sulphuric acid in a concentration of 99.8 to104.5% to a concentrated sulphuric acid solution for the para-aromaticpolyamide.